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1. Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

   https://doi.org/10.1038/s41598-018-32848-7  

   Hong, Qi-Jun; Ushakov, Sergey V.; Kapush, Denys; Benmore, Chris J.; Weber, Richard J. K.; van de Walle, Axel; Navrotsky, Alexandra (October 2018, Scientific Reports)

   Abstract Structure and thermodynamics of pure cubic ZrO₂and HfO₂were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automatedab initiomolecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO₂and 0.87 ± 0.08% for HfO₂), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO₂and 61 ± 10 kJ/mol for HfO₂, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO₂and HfO₂are similar and reach (4 ± 1)·10⁻⁵/K from experiment and (5 ± 1)·10⁻⁵/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO₂and ZrO₂, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO₂is an excellent oxygen conductor, perhaps even better than YSZ. 

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2. Crystal Structure Solution and High Temperature Thermal Expansions of NaZr2(PO4)3-type Materials

   Benjamin S. Hulbert, Julia E. (February 2024, Acta crystallographica Section B Structural science)

   Edited by O. V. Yakubovich, Moscow State University (Ed.)

   The NaZr2P3O12 family of materials have shown low and tailorable thermal expansion properties. In this study, SrZr4P6O24 (SrO 4ZrO2 3P2O5), CaZr4P6O24 (CaO 4ZrO2 3P2O5), MgZr4P6O24 (MgO 4ZrO2 3P2O5), NaTi2P3O12 [1 2(Na2O 4TiO2 3P2O5)], NaZr2P3O12 [1 2(Na2O 4ZrO2 3P2O5)], and related solid solutions were synthesized using the organic–inorganic steric entrapment method. The samples were characterized by in-situ high-temperature X-ray diffraction from 25 to 1500 C at the Advanced Photon Source and National Synchrotron Light Source II. The average linear thermal expansion of SrZr4P6O24 and CaZr4P6O24 was between  1  x 10 -6 per  °C and 6  x 10 -6 per  °C from 25 to 1500 °C. The crystal structures of the high-temperature polymorphs of CaZr4P6O24 and SrZr4P6O24 with R3c symmetry were solved by Fourier difference mapping and Rietveld refinement. This polymorph is present above  1250 °C. This work measured thermal expansion coefficients to 1500 °C for all samples and investigated the differences in thermal expansion mechanisms between polymorphs and between compositions. 

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3. Epitaxial stabilization versus interdiffusion: synthetic routes to metastable cubic HfO ₂ and HfV ₂ O ₇ from the core–shell arrangement of precursors

   https://doi.org/10.1039/c9nr07316g  

   Fleer, Nathan A.; Thomas, Melonie P.; Andrews, Justin L.; Waetzig, Gregory R.; Gonzalez, Oscar; Liu, Guan-Wen; Guiton, Beth S.; Banerjee, Sarbajit (November 2019, Nanoscale)

   Metastable materials that represent excursions from thermodynamic minima are characterized by distinctive structural motifs and electronic structure, which frequently underpins new function. The binary oxides of hafnium present a rich diversity of crystal structures and are of considerable technological importance given their high dielectric constants, refractory characteristics, radiation hardness, and anion conductivity; however, high-symmetry tetragonal and cubic polymorphs of HfO 2 are accessible only at substantially elevated temperatures (1720 and 2600 °C, respectively). Here, we demonstrate that the core–shell arrangement of VO 2 and amorphous HfO 2 promotes outwards oxygen diffusion along an electropositivity gradient and yields an epitaxially matched V 2 O 3 /HfO 2 interface that allows for the unprecedented stabilization of the metastable cubic polymorph of HfO 2 under ambient conditions. Free-standing cubic HfO 2 , otherwise accessible only above 2600 °C, is stabilized by acid etching of the vanadium oxide core. In contrast, interdiffusion under oxidative conditions yields the negative thermal expansion material HfV 2 O 7 . Variable temperature powder X-ray diffraction demonstrate that the prepared HfV 2 O 7 exhibits pronounced negative thermal expansion in the temperature range between 150 and 700 °C. The results demonstrate the potential of using epitaxial crystallographic relationships to facilitate preferential nucleation of otherwise inaccessible metastable compounds. 

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4. Strong π-stacking causes unusually large anisotropic thermal expansion and thermochromism

   https://doi.org/10.1073/pnas.2106572118  

   Dharmarwardana, Madushani; Otten, Brooke M.; Ghimire, Mukunda M.; Arimilli, Bhargav S.; Williams, Christopher M.; Boateng, Stephen; Lu, Zhou; McCandless, Gregory T.; Gassensmith, Jeremiah J.; Omary, Mohammad A. (October 2021, Proceedings of the National Academy of Sciences)

   π-stacking in ground-state dimers/trimers/tetramers ofN-butoxyphenyl(naphthalene)diimide (BNDI) exceeds 50 kcal ⋅ mol⁻¹in strength, drastically surpassing that for the^*3[pyrene]₂excimer (∼30 kcal ⋅ mol⁻¹; formal bond order = 1) and similar to other weak-to-moderate classical covalent bonds. Cooperative π-stacking in triclinic (BNDI-T) and monoclinic (BNDI-M) polymorphs effects unusually large linear thermal expansion coefficients (α_a, α_b, α_c, β) of (452, −16.8, −154, 273) × 10⁻⁶⋅ K⁻¹and (70.1, −44.7, 163, 177) × 10⁻⁶⋅ K⁻¹, respectively. BNDI-T exhibits highly reversible thermochromism over a 300-K range, manifest by color changes from orange (ambient temperature) toward red (cryogenic temperatures) or yellow (375 K), with repeated thermal cycling sustained for over at least 2 y. 

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5. High-pressure experimental study of tetragonal CaSiO3-perovskite to 200 GPa

   https://doi.org/10.2138/am-2021-7913  

   Sun, Ningyu; Bian, Hui; Zhang, Youyue; Lin, Jung-Fu; Prakapenka, Vitali B.; Mao, Zhu (January 2022, American Mineralogist)

   Abstract In this study, we have investigated the crystal structure and equation of state of tetragonal CaSiO3-perovskite up to 200 GPa using synchrotron X-ray diffraction in laser-heated diamond-anvil cells. X-ray diffraction patterns of the quenched CaSiO3-perovskite above 148 GPa clearly show that 200, 211, and 220 peaks of the cubic phase split into 004+220, 204+312, and 224+400 peak pairs, respectively, in the tetragonal structure, and their calculated full-width at half maximum (FWHM) exhibits a substantial increase with pressure. The distribution of diffraction peaks suggests that the tetragonal CaSiO3-perovskite most likely has an I4/mcm space group at 300 K between 148 and 199 GPa, although other possibilities might still exist. Using the Birch-Murnaghan equations, we have determined the equation of state of tetragonal CaSiO3-perovskite, yielding the bulk modulus K0T = 227(21) GPa with the pressure derivative of the bulk modulus, K0T′ = 4.0(3). Modeled sound velocities at 580 K and around 50 GPa using our results and literature values show the difference in the compressional (VP) and shear-wave velocity (VS) between the tetragonal and cubic phases to be 5.3 and 6.7%, respectively. At ~110 GPa and 1000 K, this phase transition leads to a 4.3 and 9.1% jump in VP and VS, respectively. Since the addition of Ti can elevate the transition temperature, the transition from the tetragonal to cubic phase may have a seismic signature compatible with the observed mid-lower mantle discontinuity around the cold subduction slabs, which needs to be explored in future studies. 

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